DESCRIPTION (the applicant's description verbatim): This laboratory's objective is to identify support and specification factors that are secreted by early embryonic endoderm cells, which regulate heart development. We have recently determined that endoderm strongly expresses the vitamin A transport proteins transthyretin (TTR) and retinol binding protein (RBP). Mapping sites of TTR and RBP protein and mRNA expression in the embryo has revealed that these factors are associated with the heart forming region in general and with definitive myocardial structures in particular. Because the developing heart is highly sensitive metabolic products of retinol, particularly retinoic acid (RA), it is hypothesized that RBP derived from both endoderm and definitive myocardial cells is necessary to regulate retinol delivery to myocardial structures during their development. Deletion of the RBP gene via homologous recombination will be used to test this hypothesis in a variety of murine embryonic models. (1) The first aim will use targeted ES cells to examine the effect of the RBP-/- mutation during cardiogenesis in embryoid bodies. (2) The second aim will utilize chimeric embryos prepared by combining homozygous-deficient RBP-/- ES cells with morula-stage Rosa26 wild-type embryos that constitutively express b-galactosidase. The exclusion of RBP-/- mutant cells from the heart of embryos whose organs are otherwise chimeric would indicate an autonomous requirement for the RBP gene in heart development. (3) The third aim will directly establish the necessity of the RBP gene for heart development by producing RBP-/- embryos from RBP-/- ES cells using the technique of tetraploid aggregation. Finally, (4) a line of RBP-null mice will be prepared to extend these studies. Results from these experiments will elucidate the dependence of the developing myocardium on carefully regulated levels of vitamin A and its product, retinoic acid (RA). These findings will further contribute to an elucidation of the mechanisms which underlie congenital heart defects as well as the reasons why the adult myocardium is incapable of repair.